1. Field of the Disclosure
Embodiments disclosed herein relate generally to thrusters that apply a force to a drill bit during the drilling of an underground formation. In another aspect, embodiments disclosed herein relate to control of a thrust force applied to a drill bit by a thruster. More specifically, embodiments disclosed herein relate to controlling a pressure or differential pressure across a thrust piston, thereby limiting the maximum applied thrust force.
2. Background
Hydraulic thrusters are used for applying a force to an earth boring drill bit, independent of the drill string weight. Although thrusters may be used during vertical or inclined drilling, hydraulic thrusters are generally advantageous in horizontal or near-horizontal drilling. During horizontal drilling, especially in long horizontal sections, a significant portion of the weight of the drill stem is directed toward the low side of the hole, detracting from the weight available for bit thrust. Hydraulic thrusters allow for extended reach drilling, the drilling of multiple horizontal wells from a single platform, decreasing the drilling costs associated with producing reservoirs that are offshore, in arctic regions, mountains, or near large cities.
The thruster is a telescoping tube arrangement that allows the drill bit to advance while the tubing string is supported in a rather stationary position at the surface. When the thruster has advanced its full stroke, or a notable portion thereof, the drill string is lowered from the surface, causing the upper end of the thruster to slide down and reset the thruster for the next stroke. When the drilling kelly or the stand being drilled down by the top drive reaches the drill rig floor, circulation is interrupted and another piece of tubing is added to the string at the surface or the coiled tubing is further unspooled into the wellbore. This drilling procedure also causes the thruster to reset.
Hydraulic thrusters are described in, for example, U.S. Pat. No. 4,615,401 and patents referenced therein (U.S. Pat. Nos. 3,298,449, 3,399,738, 3,797,589, 3,799,277, 4,040,494, and 4,040,495), each of which is assigned to the assignee of the present invention, and each of which is incorporated herein by reference. In the '401 patent, the hydraulic thruster includes a mandrel and sleeve forming two expandable chambers with wall anchors annularly disposed about the sleeve responsive to a pressure differential between a chamber and the bore hole pressure. Valves and actuators are provided to extend and retract a piston between two extremes of relative axial motion between the mandrel and sleeve.
Hydraulic thrusters are also described in U.S. Pat. No. 5,205,364. In the '364 patent, the hydraulic thruster includes a telescoping assembly for transmitting hydraulic force to the drill bit at the bottom of the tool. The internal hydraulic characteristics of the tool may be varied to vary the force exerted during extension and retraction of the telescoping assembly. The hydraulic characteristics are varied by varying the surface area within the drill tool on which the flow of drilling mud may act when producing the hydraulic force.
Other patents describing thrusters or equipment for controlling force or weight on the bit, for example, may include U.S. Pat. Nos. 5,316,094, 6,601,652, 7,156,181, 5,476,148, 5,884,716, 5,806,611, 6003,606, 6,230,813, and 6,286,592, and U.S. Patent Application Publication No. 20010045300.
Referring now to FIG. 1, a simplified cross-sectional view of a prior art thruster 1 is illustrated. Thruster 1, shown in the retracted position, may include an inner mandrel assembly 2, which may include one or more tubular components. Threads 3 may connect inner mandrel assembly 2 to the lower drill stem (not shown) toward the bit (not shown). Threads 4 may connect inner mandrel assembly 2 to the upper drill stem (not shown). Inner mandrel assembly 2 is disposed in and is axially movable with respect to outer tubular assembly 5. One or more anchor pistons 6 may be provided to anchor thruster 1 with respect to the hole wall (not shown). Drilling fluid supplied to the bore 2A of inner mandrel 2 and to the drill bit (not shown) defines a high pressure area, and drilling fluid returning from the bit in the annulus 7 formed between the outer tubular assembly 6 and the hole wall defines a low pressure area. During thrusting, thrust mechanism 8 may be used to allow the high pressure drilling fluid into chamber A, allowing fluid in chamber B to escape to annulus 7, thus creating a pressure differential across thrust mechanism 8, causing the inner mandrel 2 to advance in direction 9, and putting weight on the bit corresponding to the thrust force generated by the pressure differential.
A cross-sectional view of a simplified thrust mechanism 8, which may be used in the thruster of FIG. 1, is illustrated in FIG. 2. Thrust mechanism 8 may include an inner tubular member 12 and an outer tubular member 14. Drilling mud flowing through the bore 16 of inner tubular member 12 flows to the drill bit (not shown), and returns to the surface via annulus 18, such as between outer tubular member 14 and a drill casing (not shown). When mud is flowing through thruster 1 (FIG. 1), bore 16 is at a higher pressure than fluid returning through annulus 18. A piston 20, separating a first fluid chamber 22 and a second fluid chamber 24, may transmit an axial force 26 to inner tubular member 12. During thrusting, high pressure drilling mud flows from the bore 16 of the thruster 1 through inlet 28 into first fluid chamber 22, displacing fluid in second fluid chamber 24 through outlet 30 and causing the inner tubular member 12 to advance in the direction of axial force 26. The axial force 26 that is generated, for example, may be a function of the differential pressure between the fluid in bore 16 and annulus 18.
Many of the patents cited above use such a differential pressure to control the force applied to the drill bit. Although not shown in FIG. 2, thrust mechanism 8 may typically include ball valves, springs, and other mechanisms to control the flow of fluid into and from the high and low pressure chambers, respectively, during thrusting and retraction. One problem associated with this type of thruster technology includes the need to estimate the pressure and required thrust force prior to drilling. The thruster and the associated internal parts are generally selected and fabricated at the surface prior to installation on a drill string, and many of the parts used to control fluid flow, such as springs, check valves, flow orifices, and others, are sized and selected based on an expected downhole pressure.
Often, an actual downhole pressure differs from the expected downhole pressure. The difference between actual and expected downhole pressure, even by as little as 25 or 50 psi, may result in ineffective control of the force applied to the drill bit by the thruster, often as a result of the thrust mechanism fully opening or fully closing. Additionally, fluctuations in pressure drop across the bit and changes in the weight of the drilling fluid used (and hence bore pressure) may also result in ineffective control of the force applied to the drill bit by the thruster. Ineffective thruster control may lead to stalls, motor wear, stuck bits, and inefficient rate of penetration, among other problems known to those skilled in the art.
Various methods and apparatus have been proposed to compensate for a change in downhole conditions and to minimize the effect such changes have on the operation of the thruster and the force applied to the drill bit. For example, a pressure-modulation valve assembly is disclosed in U.S. Pat. No. 6,102,138. Such methods and apparatus unnecessarily increase the total number of drilling components of a drill string, where the additional apparatus may be prone to failure or malfunction due to various conditions encountered during drilling.
Accordingly, there exists a need for a thruster that may control the force applied to a drill bit independent of the downhole pressure or the pressure drop across the motor and bit. Additionally, there exists a need for a thruster that may control the force applied to the bit independent of the pressure of the supplied drilling fluid.